The present invention relates generally to a battery charger circuit, and, more particularly, to a circuit arrangement to reduce standby current.
Battery chargers for electric shavers, mobile phones, fax machines, cordless phones, and other electronic devices generally include a relatively simple, low cost, low frequency (i.e., 50/60 Hz) transformer circuit. Typically, the transformer circuit has a simple diode rectifier at the output of a secondary winding. Although this type of transformer circuit may be adequate for certain applications, these transformer circuits consume a considerable amount of electrical energy without a load being coupled to the output. This condition is usually referred to as standby.
The consumption of electrical energy during standby results from the non-ideal input characteristics of the transformer (i.e., magnetizing inductance) that permit magnetizing current to flow in a primary winding of the transformer circuit when connected to a power source. This magnetizing current flows through the primary winding of the transformer and also induces magnetic flux within the iron core of the transformer, both of which usually have power losses. In a typical situation, the losses of a transformer circuit during standby may exceed one (1) watt of power.
To prevent this standby loss, battery charger circuits have been developed with common high frequency inverter circuits. These circuits often utilize sophisticated integrated control circuits which can reduce power consumption during standby. However, these battery charger circuits are relatively expensive and have limited consumer acceptance.
The present invention provides a relatively simple, low cost battery charger circuit to reduce power consumption during standby. The battery charger circuit controls the current supplied to a transformer. The battery charger circuit determines the magnitude of the value of the current flowing to a primary winding of the transformer. When the value of the primary current includes a load current and a magnetizing current, the battery charger circuit continues to supply current to the primary winding of the transformer to charge the load. When the value of the primary current only includes a magnetizing current, the primary current is prevented from flowing to the transformer for a pre-set time or interval. Accordingly, the magnetizing current only flows to the primary winding of the transformer for a relatively short period of time, thereby reducing power consumption during standby.
A battery charger circuit in accordance with the present invention includes a switching element in series with a primary winding of a transformer. Control circuitry renders the switching element conductive during an on period so as to produce a current through the series arrangement and renders the switching element non-conductive during an off period.
Another battery charger circuit in accordance with the present invention includes a triggerable electronic switch to provide charging current when a load is present and to shut off the charging current when there is no load. A threshold detector is coupled to the triggerable electronic switch to trigger the triggerable electronic switch when the voltage of the threshold detector reaches a predetermined value.
Another battery charger circuit in accordance with the present invention includes a triggerable electronic switch having a gate terminal and first and second terminals through which an alternating current is supplied. The triggerable electronic switching allows current to flow to the transformer when there is a load and prevents current from flowing to the transformer when there is no load. A sensing element is coupled between the second terminal of the triggerable electronic switch and a primary winding of the transformer, and a capacitive element is coupled to the second terminal of the triggerable electronic switch. An input of the threshold detector is coupled to the gate of the triggerable electronic switch to allow current to flow to the gate terminal of the triggerable electronic switch at a predetermined voltage of the capacitive element. A load detector circuit is coupled to the sensing resistor. A switching element allows the capacitive element to be charged when a first current is sensed by the load detection circuit and to prevent the capacitive element from charging when a second current is sensed by the load detection circuit.
A method in accordance with the present invention includes the steps of providing current to charge a capacitive element, rendering a threshold switch conductive when the voltage of the capacitive element reaches a predetermined voltage to supply a latching current through threshold switch, and rendering a triggerable electronic switch conductive in response to the latching current to provide a primary current. The method also includes the steps of sensing the primary current at predetermined intervals to determine whether there is a load, maintaining the triggerable electronic switch in a conductive position when the value of the primary current includes a load current plus a magnetizing current, and opening the triggering electronic switch to interrupt the primary current for a predetermined interval when the value of the primary current only consists of the magnetizing current.
Another method in accordance with the present invention includes the steps of defining a current path in series with a first winding of a transformer, and sensing current flowing through the current path. The method also includes the steps of allowing the current to flow to the first winding when the current includes a load current and a magnetizing current, and preventing the current from flowing to the first winding when the current includes only a magnetizing current.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.
The invention, together with attendant advantages, will be best understood by reference to the following detailed description of the preferred embodiment of the invention, taken in conjunction with the accompanying drawings.